Hey bro, is your gal a commited bodybuilder or competing in WPD? If not...if she is a normal gal, just looking for an edge, deca is a bad idea.
Ok, that said, injectable oil based steroids do tend to cause a decent deal of pain, especially when injected into virgin muscle. As you get used to injections, they get easier and the pain gets to be less. Some suggestions would be to inject slowly and make sure that the needle is stable and not moving around. Also, delts are not as good of a site as glutes. Glutes are bigger muscles with more room to let the oil disperse.
If you are in question about technique check this out :
http://juicedmuscle.com/showthread.php?49-A-Guide-to-Safe-Injection-Technique