I am glad that you decided to ask for a neutral 'third party' opinion before you made this request of your girlfriend.
Any cosmetic procedure such as 'lip enhancement' should be done only if the actual individual chooses to have the procedure.
It should not be done to please anyone else, or fulfill another person's wish or desires.
It appears that if your girlfriend did not like her lips, she may have had the lip enhancement already.
In our society, we have as a whole, become too caught up with cosmetic procedures such as lip enhancements, etc...
In my opinion, people need to focus more on 'internal beauty,' rather than focusing so much time on pursuing physical beauty perfection.
If you, and your girlfriend, are in an otherwise happy, and emotionally healthy relationship, then this is what should really matter. Not the size of her lips.
It appears this is more your issue than your girlfriend's.
Perhaps you should consider seeking some professional counseling. Many hospitals and healthcare facilities have a 'referral services.' Also, you may want to ask your 'primary care physician' if he, or she, could recommend a 'mental health care professional.' This may be a good start.
If financial affordability is an issue, be certain that you mention this. Perhaps, the 'referral service,' or even your own 'primary care physician,' can recommend a competent 'mental health professional,' who would charge you a more reasonable fee.
(*Policies regarding referrals may vary in referral services, and among physicians.)
Perhaps they may also be able to recommend a 'not-for-profit counseling center' in the area where they would charge a more reasonable fee.
I would suggest that before you ask your girlfriend about having her lips enhanced, you consider discussing this matter further with a competent 'mental health professional.'
I suggest this, because maybe there are other 'hidden emotional issues' that may need to be addressed, that you may not be aware of at this time.
Good luck!, and best wishes.
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Addition to answer from 10-12-09.
I meant to include in my answer that it is a 'good indication' that you are seeking 'expert advice' before asking your girlfriend to have her lips enhanced. It is a good thing that
you appear to be mindful of her feelings, as you do not want to offend her.
This indicates to me that perhaps your relationship has potential.
As I stated above, I would still suggest that you consider talking about this further
with a mental health professional. Perhaps it can be beneficial.
Again, good luck!, and best wishes.