Comparability-wise it should be alright. Guppies can be a little nippy, and sometimes bettas are known to go after them because of their bright colors, but females tend to generally be a tad bit more docile then males.....so I'd say so long as there was enough cover in the 10 gallon, you shouldn't have too many issues.
However, your tank I'd say is rather under-stocked as opposed to over-stocked. Neons are schooling fish and require groups of at least 5. Guppies are also a social species and would probably appreciate at least a couple more of their own species.
With proper groups of both fish, your tank would be about fully stocked, but since you stated you were only considering adding your girl temporarily/in an emergency I'd still say it would be alright for a short while in a pinch.