If a soldier is medboarded out the Army, but wants to join the Air Force, would they have to pay back any funds they received from the Army?

The only way a soldier who has been MEB out of the Army to rejoin the military is if they were TDRL and not PDRL. Even if they were TDRL, it’s almost impossible for Army to transfer to Air Force because the Air Force has a prior service quota and it’s normally only open to a select number of MOS

It would literally be several paragraphs explaining all the reasons this wouldn't work out, several already pointed out by SSG (Join to see) and SSgt Christophe Murphy, based on the type of separation, what component they are joining, and why they were separated

Long story short, unless they were medically separated - not medically retired - and joining the AF NG, it's not going to happen.

Given the MEB's result, and the current manpower needs, the Air Force can pick and choose its new recruits. After a banner year of growth that surpassed its recruiting goals, the Air Force thinned the force last year, tossing out those who defied mask mandates, etc. I doubt it would take on an avoidable burden on the taxpayer, unless the prospect had a supremely important set of high tech skills to bring to the table. The chances of his acceptance are less than zero.