The other option is for you to meet them in a different country where you and they can visit. Somewhere in Europe? Both US and Ukrainian citizens can enter EU freely for 90 days.
Another option is Mexico (visa free for US citizens and eTA for Ukrainians). Or Canada (visa free for US citizens and visa required for Ukrainians).
Green cards are for living in the US. If they're not going to live in the US, they'll lose GCs eventually.