Praise be to Allah.
It is haraam for a woman to work in a mixed workplace, because of the evils and bad consequences that result from that. This has been discussed in the answers to questions no. 50398 and 1200.
It is essential to differentiate between doing haraam work or working in a haraam job, and committing haraam actions whilst doing one’s job.
The first case includes working in riba-based banks, or dealing in alcohol, drugs and lottery tickets, and other haraam things. In this case, the money that comes from that job is haraam and he has to get rid of it by spending it on various charitable causes, as has been explained in the answer to question 78289.
The second case includes doing permissible work, in which case his wages or salary from this job are permissible and there is nothing wrong with it, but he is sinning if he falls into sins that may accompany that job, such as mixing, smoking and so on. But that does not make the money he earns haraam.
To sum up:
The money that this woman earns is not haraam just because her job involves mixing, unless the work of the company itself is haraam, such as if it is a company that sells tobacco or a riba-based bank and so on.
Hence there is nothing wrong with accepting an invitation from someone who works in such a company or accepting gifts from them or benefiting from their money, but it is essential to advise her about the issue of free mixing and tell her to try to avoid it, if that is possible, or else look for another place to work where there will be none of this free mixing.
Please note that if a person’s earnings are dubious or are a mixture of halaal and haraam, there is nothing wrong with eating his food or accepting gifts from him, as the Prophet (blessings and peace of Allah be upon him) used to eat the food of the Jews and accept gifts from them.
Please also see the answer to question 87747
And Allah knows best.