An iPhone is intended (built) to be synced to exactly one iTunes library. It sounds like her iPhone is synced to a different iTunes on a different computer. Do not sync it to a second computer.
Two things to note:
1. There may well be ownership issues doing what you're trying to do regardless of which library is used. If you purchased the iTunes music, it is yours, and not hers.
2. If the music was obtained some other way (e.g., ripped from CDs), I recommend simply transferring the music to her computer and letting her sync with that machine as usual.
oh no there's no issue there, previously it was synced to our macbook and now I want to sync it to our mac computer because the macbook doesn't work anymore so thats not a problem.
i realise now i shouldn't have said my mac computer we both use it so yeah with that in mind do you know if the contacts, photos, messages etc will be wiped?
... do you know if the contacts, photos, messages etc will be wiped?
Yes... Everything will be Wiped and Replaced with what is currently on Your Mac.
SYNCING with iTunes
See here > http://support.apple.com/kb/HT1386
From Here > http://www.apple.com/support/iphone/syncing/
You may find this information of interest...
Have a read here...
And See Here...
AnaMusic is incorrect.
You may safely let iTunes delete the content from the 2nd iPhone since you already have it on the 2nd computer. Uncheck the box for syncing the Photos and it will leave the existing photos on that phone alone. Dittos for Contacts and other data. Text messages and all other content will be left intact.
Ideally, you will want to import those photos onto that computer using the Image Capture application or iPhoto. Then you will have all photos and iTunes content together on one computer in the same account.
iTunes syncing can then be done with one computer, one account, and as many iOS devices as you like. However, keep in mind that by sharing one Mac account, you both share access to the same photos and other media together. There is no privacy.