I'm lactose intolerant. The intolerance did not show up until I was well into adulthood. I suggest that the leg injury and the appearance of lactose intolerance is purely coincidental.
I've never heard of breast milk therapy for an illness or injury. I doubt that breast milk will help his leg. There is no connection between a leg injury and digestion. No connection between digestion and bones/circulation. There is a connection between calcium consumption and bone strength. He can get calcium, which may strengthen his bones, from supplements or fortified milk substitute drinks. I love rice milk which does contain calcium and Vit D.
Circulation is thru blood vessels. I suspect he has poor circulation because vessels have been constricted or destroyed. Breast milk will not repair or replace blood vessels. There is no way to replace massive destroyed circulation. The body will reroute around some destroyed vessels and I suggest that is why your husband still has his leg. But again, digestion of breast milk does not assist in making vessels.
Your doctor can be more technical in giving you an explanation. I urge you to talk with a doctor and ask this question.