He has 250. Can do CFI and/or other GA work. In the right location with some hustle you can log 80-100 hours/month (BTDT). So that's 12-18 months with some hustle and luck (ie no GA slowdown due to economy/coronavirus).

If he has a at least 20 months until UPT, he could still grab a seniority number at a regional even if he doesn't finish training.

I recommend that folks do not bang out on discretionary long-term orders in their first airline year JUST for pay and schedule convenience. That's NOT what the system is intended for and reflects poorly on the rest of us. But in a case like this, I'd say go for it... he needs to get his 121 career started and really has no choice as to when he goes to UPT. Between a rock and a hard place, and I would suggest that he not let his civilian career suffer due to his military service... that IS in keeping with the intent and spirit of the law.