Maintenance isn't for treats! It's to meet all the boring costs of raising a child, including food, heating, socks, haircuts, etc.
Parents and teens often disagree on what counts as essential. Parents will usually prioritise bills, food, clothing and will budget for upcoming expenses, etc whereas many teens would be happy to miss a couple of utility bill payments to get some really cool trainers.
By giving the money direct to the DC, you could be preventing the mother from making sensible decisions and leave her unable to meet the financial demands of raising the child in question.
I often have to tell my DC that I can't afford treats and fancy things they want as I don't have much money - it's not because I'm spending their child maintenance on frivolous things, it's because raising teens and running a home is bloody expensive and teens always think they're hard done by compared to everyone else.
I would advise DH not to change the payment setup unless he's sure that the mother is exploiting the situation - otherwise, he needs to trust that the adult responsible for his DC knows what she's doing and stop interfering.