Praise be to Allah.
If a person says “May the gods curse you”, he is committing two grave wrongs.
The first is affirming other gods alongside Allah that are able to curse – exalted be Allah far above that, for there is no true God except Allah, and no god who is able to curse except Allah.
Such words constitute disbelief in Allah (may He be exalted), and doing that by way of joking does not mean that it does not constitute disbelief (kufr). Similar to that is saying, “May the curse of Hubal – or of the mountains – be upon you.”
Allah (may He be exalted) says (interpretation of the meaning):
“And if you ask them, they will surely say, ‘We were only conversing and playing.’ Say, ‘Is it Allah and His verses and His Messenger that you were mocking?’
Make no excuse; you have disbelieved after your belief. If We pardon one faction of you - We will punish another faction because they were criminals”
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: Thus it is clear that they were disbelievers because of their words, even though they did not believe that those words were true.
End quote from as-Saarim al-Maslool (p. 523).
And he (may Allah have mercy on him) said: To sum up, whoever says or does something that constitutes disbelief becomes a disbeliever thereby, even if he did not intend to become a disbeliever, because no one intends to be a disbeliever, except as Allah wills.
End quote from as-Saarim al-Maslool (p. 184).
The second wrong action is cursing a specific person; the more correct scholarly view is that this is haraam, even if it is directed at a disbeliever or a evildoer, so how about if it is directed at a Muslim who is not an evildoer? See the answer to question no. 36674.
The Prophet (blessings and peace of Allah be upon him) said: “The believer is not a slanderer, one who curses a great deal, one who indulges in obscenity or who in engages in foul speech.” Narrated by at-Tirmidhi (1977); classed as saheeh by al-Albaani in Saheeh at-Tirmidhi.
At-Tirmidhi also narrated (2019) that Ibn ‘Umar said: The Prophet (blessings and peace of Allah be upon him) said: “The believer is not given to cursing.” Classed as saheeh by al-Albaani in Saheeh at-Tirmidhi.
This implies that it is forbidden to curse a great deal, because faith cannot be undermined except by omitting to do something obligatory or doing something prohibited.
Hence one of the causes of a person being admitted to Hell is that he curses a great deal, as al-Bukhaari (304) and Muslim (80) narrated that Abu Sa‘eed al-Khudri said: The Messenger of Allah (blessings and peace of Allah be upon him) came out on [Eid] al-Adha or [Eid] al-Fitr to the prayer place, and he went to the women and said: “O women, give in charity, for I have seen that you are the majority of the people of Hell.”
They said: Why, O Messenger of Allah?
He said: “You curse a great deal and are ungrateful to your husbands. I have never seen anyone so lacking in reason and religious commitment, more overwhelming to a man of resolve than one of you.”
Muslim (2599) narrated that Abu Hurayrah said: It was said: O Messenger of Allah, pray against the mushrikeen. He said: “I was not sent as an invoker of curses, rather I was sent as a mercy.”
Muslim (2597) also narrated from Abu Hurayrah that the Messenger of Allah (blessings and peace of Allah be upon him) said: “It is not appropriate for a siddeeq (sincere and true believer) to be an invoker of curses.”
Muslim (2598) narrated from Abu’d-Darda’: I heard the Messenger of Allah (blessings and peace of Allah be upon him) saying: “Invokers of curses will not be intercessors or witnesses on the Day of Resurrection.”
An-Nawawi (may Allah have mercy on him) said in Sharh Muslim: It is not appropriate for a siddeeq to be an invoker of curses, or for invokers of curses to be witnesses or intercessors on the Day of Resurrection – this is a deterrent against cursing, and a reminder that whoever has this attitude will not have these good characteristics, because what is meant by cursing in the context of offering supplication is praying that someone be cast far away from the mercy of Allah (may He be exalted).
Praying for such a thing is not a characteristic of the believers whom Allah (may He be exalted) described as being merciful to one another and cooperating in righteousness and piety, and He described them as being like a structure, parts of which support other parts, and as being like a single body, and said that the believer loves for his brother that what he loves for himself.
So if someone prays against his Muslim brother, and prays that he be cursed – which means being cast far away from the mercy of Allah (may He be exalted) – this is the ultimate severing of ties and turning one’s back on one another, and this is the ultimate of what the Muslim wishes for the disbeliever and prays against him. Hence it says in the saheeh hadith: “Cursing a believer is like killing him,” because the killer cuts him from the benefits of this world, and the one who curses him wants to cut him off from the bliss of the hereafter and the mercy of Allah (may He be exalted).
It was also said that what is meant by cursing a believer being like killing him is in terms of sin, and this is more likely to be correct. End quote.
If the matter is as described in these hadiths, then how can the Muslim accept this description for himself, and miss out on the status of being a siddeeq or a witness or an intercessor on the Day of Resurrection?
Thus you will understand that this kind of cursing is evil, and if there is added to it that which is more serious, namely disbelief (kufr) – may Allah keep us safe and sound – then in that case your friend should repent to Allah (may He be exalted) and give up all of that.
And Allah knows best.