The details you are unsure of are important. The man and woman need to make an appointment with their local pastor, discuss their situation, and follow his advice regarding next steps.
You don’t mention whether these individuals are currently practicing the faith, attempting to return, or non-practicing. Sounds like they aren’t practicing and don’t care to be practicing, so they may not want to take any steps to get married in the Church.
They would both need to be declared free to marry. This happens by examining their prior marriages. In the case of the man who was not married in the Church, if he was married outside Catholic form without a dispensation, then he did not have a valid marriage and would turn in some paperwork and that would be it. There is not enough information regarding the woman to say much, if she was married outside the Church, same thing applies-- lack of form. If she was married in the Church, then she needs to talk to her pastor about a decree of nullity.
If they are both free to marry, I am not sure what makes you think they cannot marry in the Catholic Church.
If they do not resolve their prior marriages first or get married outside the Catholic Church, then yes they are committing adultery.
I am not sure why you think you have responsibility for the decisions of two grown people?
You should encourage them to go talk to their pastor, and refrain from speculating out how “easy” or “hard” it might be to rectify their situation since you don’t know all the details of the prior marriages. Simply encourage them to talk to their pastor.
ke answered correctly, of course, but I’m going to give you a slightly different perspective on the answer than she did. Here, you actually asked two questions: (a) are they committing adultery? and (b) are they necessarily in a state of adultery forever?
So, just to clarify: yes, based on your description, it seems that they are currently in a state of adultery. No, they aren’t necessarily going to be committing adultery forever – they can get out of that state of sin by talking with their priest, getting any necessary decree(s) of nullity, and marrying in the Church.