How to confront husband

My bestie has been having some suspicions about her husband (he lives in another country) cheating. His sister lives with him and she asked the sister if her husband has been cheating. The sister told her there was a woman he brought to the home a few months back and she stayed the night in their bedroom. The sister also said that her brother cheats and she does not trust him. She does not want to be the reason they divorce so my friend can’t let her husband know how she found out the info. She feels she needs to have this proof prior to filing the papers but not sure how to go about it. What wound you do ?