She really loves him. That is adorable.
First panel - Jon, Liz, and Garfield are minding their own business, when Mr. Muscles come up, and asks, "Excuse me, is that your girlfriend?" Jon says, "Yes."
Second panel - KICK
Third Panel - The only change from the first is that Jon is covered in sand.
Mr. Muscles: Hi, I'm Rolf.
Liz: Hi, I'm Get Lost.
Garfield (via thought bubble): And I'm with Mr. Sand Face over there.
Jon and Liz's relationship has always been adorable, once she reciprocated and started dating him and kissing him. This just cements the awesomeness. She loves Jon for who he is as a person, she doesn't care about Rolf.