What you need to act as sponsor is:
1. Proof of your relationship, records of regular contact, details of your visits to Thailand with evidence that you were actually together - i.e. photos, evidence of any joint travel etc. etc.
2. Evidence of your home showing its is adequate for the use intended and, if its not yours - your parents for example, permission from the owner/registered tenant for your friend to stay.
3. You do not necessarily need to have a 'lump of money' in your bank account. What you need to do is convince an ECO that you have an amount sufficient to cover all the costs of her visit and that must be evidenced. If you are in regular employment you need to show proof of income and that the amount needed to cover the cost of your friend's visit is free and available for that purpose - i.e. savings. If you are not in regular employment or cannot prove your income then its better if you do have a substantial amount in a savings account. Beware of opening an account and putting a large amount in it then making the visa application straight away.
This is all from memory, I believe there are official guidelines on sponsorship.