Has anyone had this injury and any advice on how best to treat?
Did your arm go numb? Mine did and it really worried me!
This happened to me whilst dynamically latching a 3-finger pocket with my right hand back in January. I had 4 weeks where I could hardly hold anything and then started climbing easily after that. It's still not 100% (probably about 95%) now but it's not holding me back.
In the initial stages I used cold-treatment techniques to reduce the pain -- some info here
http://onlineclimbingcoach.blogspot.com/search/label/InjuriesAfter the initial stages (2-3 weeks of no climbing) I just changed how I climbed and started using different hold types (crimps, half-crimps etc.) which didn't affect the flexor so much -- mine hurt more when using open-handed grip positions.
Not sure if that helps but to sum up, once the pain subsides then start climbing easier stuff and use crimps rather than open-handed grips until it gains strength again. Expect it to be slightly weaker than normal for ~6-8 months perhaps?
To be honest though I've not really been that pro-active with my recovery and I ended up just climbing differently. I'm sure others will post more specific rehab which will speed things up.