Can I substitute grapefruit for lemon in this?

Definitely will work, though my first substitution would probably be orange

Absolutely! Any citrus should work. However, bear in mind that a grapefruit is likely to have a great deal more juice in it than a lemon (the recipe calls for 1 lemon). After some searching, I found that the average lemon contains about 3 tablespoons of juice. So, I would use 3 tablespoons grapefruit juice plus the grated rind of half the grapefruit.