So here, she implied "he's getting hot", but I don't understand what's that supposed to mean. Does she mean that he's sweating more and more because he's feeling too warm?

Rebecca

No, it's a joke. She's joking that she seems someone who is very attractive, so she feels hotter than usual. You might sometimes hear someone use the expression "is it getting hot in here or is it just me/him/her/etc." to express that they have seen someone attractive. Ex: Wow, look at all the people at this party. Is it getting hot in here or is it just me? Ex: It's getting hot in here. Look at that guy!