This article said that Rob Lowe made an unaccredited appearance on the Facts of Life. If it's the 3rd show of Season 2, then it would be where Jo goes to the dance at Bates Academy and Blair's old childhood friend attacks her.

https://www.lakecountrycalendar.com/columns/column-for-women-the-facts-of-life-dont-really-change/?fbclid=IwAR0RQaH67vROoqp10IE3eBy9_V22_2_PmohaTjeB3WV1VOY_rG0sM0PUYHo

Nope, no way did he appear in that. That tall slim blond guy is the only guest star in that episode.

Alternatively, they could mean the third show taped in season 2, which is "Shoplifting". If so, he would be an uncredited patron at Harrison's department store*, but I don't recall seeing any teenage boys there.

*(coincidentally, Blair's old childhood friend who tries to use Jo is named Harrison, lol)

ROB LOWE was definitely not on FOL: i'v watched all episodes and i didn't see him!