Tell her the following. The simplest spell is to go for a long bath, lit with candles. After the bath, towel off, brush hair, teeth, put on a little makeup, and some new lingerie.
After this she should come back to you, and you should tell her how beautiful she looks.
It's more of a ritual than a spell, but when a girl takes really good care of herself, and you give her compliments and make her feel loved, she'll feel just like she did a glamor spell on herself.