EDIT: His car is classed in DS per the 2019 SCCA Rulebook.
I think this would be the ticket to a fast DS(D-street) classed V6 Mustang:
Bilstein(keep the stock springs per the rules)
18x8" lightweight wheels
255/45R18 200tw tires
Snell SA2015 helmet
CG Lock(for the seatbelt to hold him in place)
Blue Painters Tape or a set of numbers/class letters from Trackdecals.com
Its usually $35-40 to do an autocross event. Sign him up for the local SCCA events and have fun. Once he gets the feel of the car, the power delivery and rotation he can start fine tuning with the adjustable Koni dampers along with maybe a larger front sway bar to get it to rotate more(rules allow 1 sway change in street classes).
Fixed. I'd also worry about a 255 on an 8" rim, maybe a 245 would fit better as far as sidewall stability is concerned?