Is the amount she entered higher than the amount of non gifted coins she has? Usually the first time you tip, the amount is automatically set to 50 coins if I remember correctly. If she has 225 coins for example, of which 200 are those originally gifted ones, then she can tip 25 coins. Tell her to tap the amount in the lower right corner on your "tip creator" screen, and select "other". Then she would have to enter the amount of coins she is able to tip.
For (I am assuming, having observed the transactions on my revenue page when the feature was new) transaction related reasons you cannot tip less than 25 coins at once.