Friend's Girlfriend Issues: Crushing Hard and Unsure What to Do!

I'm a 25-year-old guy in a bit of a sticky situation with my friend's girlfriend. Here's the deal:

• My friend and I have been close for 15 years, but I recently reconnected with him after being distant for a while.

• He's been dating this amazing girl for a while now. She's smart, driven, and super into him.

• The problem is, there's a lot of friction between them. He's got a huge ego and a short temper, and she often feels ignored and unappreciated.

• I've been trying to mediate their issues, but recently I've developed feelings for her. It's getting harder to suppress them.

• I'm single and have struggled with dating in the past due to some personal issues.

• I don't want to hurt my friend or jeopardize our relationship, but I also can't ignore my feelings anymore.

What should I do? Any advice would be greatly appreciated.