Can't prove it . . .

I agree to confront your husband, but leave the friend out of it at first. You might be onto something but maybe it's not her. But, maybe it is. Honestly, the biggest indicator of cheating is changed behavior. So, if he starts really doing things out of the norm. Be realistic when assessing. For example a person suddenly deciding to get in shape doesn't mean they're cheating necessarily. But going out more often with "friends", working late more often than normal without a valid sounding explanation, becoming more secretive with their laptop, tablet and phone. Showering at odd times. Carrying grooming supplies with them when they never did before. Texting more or taking personal calls more than they did before without a good explanation. Trust your gut. If it is telling you something is up you're probably right. Good luck <3

Arrange to have a friends night in, with your husband present. Excuse yourself occasionally throughout night, but at inconspicuous intervals. Ensure you have a secret camera in the area, of where your husband and friend is situated. That should record any evidence of their betrayal, if they're cheating on you.

When your husband isn't around, watch the footage to determine for yourself,
if your instincts were accurate in regards to their betrayal. If yes, pack your important belongings/essentials and leave the unimportant things you don't mind leaving. If you have trusted family and friends, contact them and ask if you could reside with them for a few days. Ensure you withdraw plenty of cash from your husbands account, if you have a joint bank account and access to it.

Open a personal account if you don't already have one and ensure you seek legal advice from a divorce attorney, take your evidential footage of their betrayal to present to your lawyer.

I'd confront your husband, then your so-called friend. If it is really going on, it is better to blow it up now than to wait.