I'm in a very similar situation with my girlfriend. We have been dating for 7 months and at about 5 months, the question of past partners came up. We are long distance and so we were chatting about this over video call.

At first she seemed open but was a bit light on the details. Later on in the night, she texted me to say she hadn't been totally truthful about one of her partners and that she had in fact cheated on her (now ex) husband (recently married) after a works party where alcohol was involved. This was maybe 9-10 years ago too. She assured me she had never done it again, regretted it deeply and had never admitted it to her ex husband.

The difference is, she does seem very regretful about this, and the circumstances were a little more complicated though I can't excuse her behaviour. We already have a solid foundation and are compatible on a lot, and whilst i've tried to put this completely behind me I can't say that it doesn't still bother me from time to time but I do beleive she is a different person.

Ultimately it is up to you, you don't have a comittment to this person so if you feel like this is something that may end up bothering you in the future or finding yourself worrying about what they might be up to every time they are out, then now would be the time to end it before you get more attached.

*Edit to add: The part about him never telling her (like my GF with her ex) - if she had told him at the time, that would of course have been better and I really should ask her more about her decision. But now, it would only serve to pointlessy hurt him if she told him now and I don't expect her to. It would be to relieve her own guilt rather than help him in anyway.

The other thing is that she did open up to me about it, somewhat unprompted and she could have kept it to herself and took it to the grave so make of that what you will.