Sounds interesting.....you could try this not exactly what your mate wants but it will work

Say he has $100 to spend on the total of all bets and i assume they are in different states even if they are not he xan still do this.

On the first selection he places 60% of the total bank in this case it is $60 if the horse wins he adds what ever he gets back to the remaining $40 and then bets 60% of that on the next horse here is example of three horse parly for the place

bank $100
first bet $60-remainder $40
winner pays $1.80 return $108 + $40 = $148
second bet 60% of $148 = $88.80 rounded up to $90-remainder $58
winner pays $1.50 retrun $135 + $58 = $193
third bet $60% of $193 =$115.80 rounded to say $115-remainder $78
winner pays $2 return $230 + $78= $308 profit $208

this could be continued for as many bets as you like if you strike a loser just use 60% of the remaining money and continue