What they're asking to do is really Just Fine.  It may or may not accomplish what they want, depending on how they do it, e.g. whether traffic will go through their other ISP or yours. The main reason it's a Not Best Practice is that it often doesn't get what the user wants, e.g. the user only has a /29, so only the aggregate advertisement from their primary ISP works anyway, or it's PA space so they'll still have to give it up if they change ISPs.  But if they've got a /24, that's big enough that most carriers will see it these days, and there's no need to clutter up the BGP ASN space if your user doesn't need the extra flexibility.

There may even be a market for ISPs to do multihoming on a cabal basis, e.g. Carrier X and Carrier Y get a /20 that they assign routes from to customers that use both of them, but only advertise the aggregate to the rest of the net.  They'd still need to handle the more specific routes internally and exchange them across their peering link, but wouldn't have to bother the rest of the net with that level of detail.