My girlfriend came to visit me at Christmas. She had a bit of hassle, I got a call from the airport and I really really failed!!!! (she is a young american, 22 years old)
1) I couldn't remember how to spell her surname (it a weird one oechsle pronounced x-ley!!)
2) she had very little money on her (being a nice gentlemen I was paying for her while she was here
3) I forgot what she was studying (they asked me I presume to make sure I wasn't lying about knowing her) (i had a fair idea, but I wasn't spot on with my answer)
4) I was NOT at the airport as planned, she arrived at like 7am which meant me being up at like 4am to meet her.... I overslept!!!!! (The person on the phone was like "you are not a nice person to visit if you cannot make it on time - I felt like such a fool haha)
For these reasons they kept her back and would not allow me to speak to her, I rang 3 times to let them know my progress (how long it would take me to get there etc etc). They asked me a few questions about her and asked if I could support her financially while she was here, which I confirmed verbally on my mobile phone, no proof was required. After 3 hours she was let "free" lol, before I even got there.
I learned my lesson. IMHO you will be fine as long as you give her a mobile to contact you on. If she does have problems, they will contact you to confirm her reasons for visiting and will probably want you to confirm you can support her while she is visiting. Although my girlfriend had a return ticket she was NOT asked to present it.
If she can get through after I failed so bad I am sure you will be fine. Good luck. This was at Manchester airport btw.