I play guitar and Piano I can't say to what standard because I play very differently on the two I am primarily a Jazz/fusion player on Guitar, but I play more classical on Piano (I'm a passable jazz pianist but not great) and I'm technically better on the Piano but I'm more unique on the guitar
Although I would say that whatever instrument she learns she will be able to play with other instrumentalists simply because that how music works, it doesn't have to be an orchestral one, but as advice I would say let her pick, as if she picks she is more likely to work hard and play better if it's an instrument she truly wants to learn, but my other piece of advice would be to wait until she is grade 5 on piano and playing at an intermediate standard at the minute she is still fairly early on in her piano training and starting two instruments here could be a bit much