What is the best supplement for a horse recovering from a tear of the deep flexor tendon?

Time, time and more Time to heal. Make sure the horse is on a good, anti-inflammatory diet with a good vitamin and mineral so they have all the tools to heal. 

Exactly what I was going to say. 

This ^^^^^^^^^^^^^^^^

Total Health Enhancement.  I had a mare shred a superficial flexor tendon.  THE Flamoxide was a big help.

Not a supplement but...... PEMF therapy for the win. I had a horse with a DDFT tear that was kinda "stuck" on healing after over a year. Two months later and PEMF sessions later, he healed it the rest of the way.

Mine had tears in both front feet. We did a rocker shoe, she had Renovo/Alpha2EQ injections, and I used the Bemer cuffs, and she was on Osteomax...

 Equiscope and THE NutraWOUND.... I have had multiple soft tissue injuries heal up in half the time with this combo as well as helped a mare grow her hoof back.

My mare had lesions on both her front DDFT and was told to do surgery and have 9 months stall rest last May. I put her immediately on NutraWOUND and got the Equiscope. Fast forward to last October she coliced and while at the vet I had them recheck her front legs. The said they didn't know what I was doing but keep it up because the lesions were gone. 

I have personally been using THE supplements and a distributor for years. You can message me if you have more questions!