How to reset visitor stay in HK (as an Ukrainian) (Hong Kong)

Hi all, i had an Ukrainian friend who come to HK for job hunting. Before he can get any job, he can only stay in. HK for 14 days as visitors. He has been travelling to Macau to reset his visitor entry of 14 days until yesterday, he was questioned by Immigration about his long stay, as he tried this back and forth trip to Macau for 2 times (I.e. 2 x 14 days = 28 days) already. At the end, he was only allowed to stay in HK for 3 days.

So my questions are, other than Macau, is that mainland china he can travel to reset the HK entry? (With least cost incurred). And if so, any risk or trouble if go thro immigration? Hope there is someone who has relevant information to share!

Many thanks
Pam