You're spoiled for choice, honestly. There are thousands of cheap,
simple, reasonably powerful decks that your dad could use. 65$ is actually a pretty healthy budget for casual.
Gonna throw a bunch of random possibilities at you now. Arranged in
rough order of complexity.
A red-green aggro deck like the one discussed
here can be pretty good for not much money.
My Vraska theme deck is cheap, and would be even cheaper if you replaced Vraska with
Spiritmonger. I expect it could beat an Ally Equip deck pretty handily.
Speaking of Equip,
my Kor tribal deck is about 20$. And it does the Equip thing very well.
Jace27's 40$ Infect deck is intended to compete in Modern, and might actually be too strong. Then again, if your dad's not too skilled that might be good.
My Wizard tribal deck might also be too strong, but I'm fond of it so give it a look and see what you think.
PS:
Relentless Rats is unfortunately quite expensive. With each of them costing about 2$, you'll have a hard time building a
Relentless Rats deck for under 65$.