I would probably do it, unless you can log on with one of her devices, because it's going to tell the IP or device name or something when the new access alert pops up once you do change the password. I have my own devices on mega lockdown, and have had them that way for a long time. I have many, many reasons for doing so, the topmost reason being that I am in an abusive relationship and I have been making plans to get out of it. If he knows I am leaving he may take steps to prevent it. I am also cheating on him, or I was anyway, but to be fair I tried to leave him on a few different occasions and he did not allow me. I thought if I cheated he would leave me instead but I'm now afraid he would hurt or kill me or the other guy. I have found things on his computer that I wish I had never seen, and I'm afraid with his blackmail tendencies that he might plant some of it on my pc, that's another reason I keep it on lockdown.
Keeping all of that in mind, if he wants to catch me, he will have to open facebook, or Skype on my computer while I am in the restroom or away making food, I only lock it when I'm out of the house or asleep. Alternatively, if I set my phone down immediately after having used it, there's about a 1 minute delay before the screen locks and the password kicks in. If I don't take my phone to the bathroom or kitchen with me, he could access my therapist, my Skype, email or facebook, assuming he has the code to unlock them. I do a good job of keeping my conversations and histories deleted, always have, but there's always that chance. I'm sure you're not abusive so the odds are good that she's cheating on you. I'm sorry.