Can we still reach loved ones with guidence after they have passed through the bardo into a rebirth?
"Can we still reach loved ones with guidence after they have passed through the bardo into a rebirth?"

If "guidance" you refer to what the book "Tibetan Book of The Death" instructed, then it would be "no". That person is in a new life form, all the possibilities during the bardo stage is conclusive now. In general saying, the bardo lasts max. 49 days. However, you may send blessings to the loved ones any time.