Word on the street is that LINDSAY LOHAN stole new boyfriend, snowboarder Riley Giles, from his fiancee - according to the girl's mother.

The 21-year-old actress confirmed reports she is dating the California-based X-Games star earlier this month, after leaving the Cirque Clinic rehab facility in Utah.

But Giles was already engaged to Utah resident Bree Tierney - and news of his relationship with the Mean Girls star has left her heartbroken.

Tierney's mother, Tess, says, "Riley just stopped calling Bree and never told her about Lindsay. "She found out by seeing photos. It destroyed her."

Not vey nice, we agree. But it looks to us as though the man's a cheater, and Bree should be happy he's out of her life if that's the way he's going to treat women. Hmph.