18 month old filly off grain 2 weeks......?

I'm boarding an 18 month old filly and she has been off of grain for 2 weeks. (I went on vacation and he tuck her somewhere else while I was gone) He brought her back today and wants her put back on grain now, so what is the best way to ease her back into her grain? She is suppose to be having 6lbs of grain a day. 3 am and 3 pm. Please help I don't want her to colic.