What does the Grooms mother have to buy (flowers, etc...)

There aren't any hard and fast rules as to what she must buy. At the least, she would buy her own dress or clothing to wear to the wedding. Generally, the groom's family also hosts/pays for the rehearsal dinner. In EZWeddingPlanner Advice (top menu bar) there are lots of topics including Paying for Expenses.

Also look in Budget, where you can track what's to be bought and who's paying for it, etc. All these items are changeable, so you can fit it to your situation. Much depends on who can best afford to pay and how much everyone wants to contribute to the wedding. Noone should go broke paying for a wedding because it's "expected".