<human>: Give me the gist of the text in a nutshell

*I know at least one of the parties involved is an active Reddit user, so I have to keep things vague.*

After meeting through a mutual friend, my (F20s) ex (F20s) and I went out for over a year. For that time, we were each other's everythings, and spent more time together than not. In the end, we broke up amicably, but things soured when she got asked out by a mutual friend (M20s) within days of us announcing our break up. They've continued dating from then on (coming on 3 years).

Though I initially gave the go ahead on my friend and my ex, and I'm at peace with their relationship, I've distanced myself from them. Just wasn't as fun to hang out with that particular dynamic. Go figure. 

Tonight I stumbled across her SO on tinder. He has pictures on there that I know for a fact are from the time they are together. 

My question is.. What should I do?? To not do anything at all feels wrong to me, but I also recognize that I might not have the most objective view on the situation. Any/all advice is appreciated!
<bot>: I found my ex's current long term boyfriend on tinder and I don't know what- if anything- I should do! Help please!