。
She blushes the moment she's around him.
Literal
She [topic-は] his [genitive-の] front [at-で] [contrast-は] quickly face [object-を] makes-red.
顔をあかくする ('to make one's face red,' i.e. blush) uses the Xを Yくする pattern, where Y is an i-adjective in adverbial form (~く). Compare with the na-adjective version Xを Yにする. The は on で here is contrastive — '(in front of him, as opposed to other contexts)' — highlighting that this happens specifically with him. The で marks the location of the action (where the blushing happens), distinct from に (location of existence). 赤い is written here in kana (あかい), a stylistic choice; both work.