Thanks, I'll try vinegar first, since I have some.

but what if they used a lot, like an idiot.

Absorbent towels. I guess if you'd always used fabric softener, you'd never know how much more effective towels are when they haven't been abused by fabric softener or drier sheets.

Polydimethylsiloxane (PDMS) is the active ingredient in most fabric softeners. However, PDMS is a silicone oil that destroys absorbent properties.

The oil in the fabric softener latches on to clothing and creates a coating. Towels absorb water, but oil repels it. When an oil coating attaches to a towel, it causes the fibers to become greasy and slippery, which hinders its absorbency. When the soapy residue of fabric softener builds up on the towel, it causes it to become stiff instead of soft.

Source: https://www.towelsupercenter.com/blog/should-you-wash-towels-with-fabric-softener/