Which pair of coaxs?

[sinister audio]

maybe you could convince her to put another amp in and replace all the coaxs and then have the cross over so it wont play that low.

[GlassWolf]

all she needs is any simple set of coaxes in back like the type S alpines, and then get some bass blockers for her front and rear speakers so only the sub handles bass. block the mids at about 100Hz and XO the sub @ about 60-80Hz.
that'll prevent blowing anything else, it's a pretty cheap fix, and overall SQ will improve enough to warran the small investment

I think she'd be happy with that fix.
you don't want to overdo it on rear speakers or you ruin the soundstage, even with a factory system.
since you're talking about 15 watts per speaker, look at sensitivity ratings instead of pwr handling.
that'll help more. the higher the sensitivity the better or louder the speaker will be with given power amounts