Any advice you can give would be greatly appreciated!
If your friend is worried about her relationship, the best thing you can do is to talk to her in person and not try to diagnose her situation from a tarot reading. You can listen and empathize with what she is going through but you should refrain from passing judgement without other confirmations of her concerns. There are a few ways to get to the bottom of her worries. She could talk to her partner about any suspicions she has and ask open questions in order to express her worries. If the partner appears shady in any way, she can resort to other ways of verification like email tracking, private investigation, phone records, etc. Ultimately, the decision should be made after methods of confirmation and the couple should work on finding a resolution together.