Michael Lohan's Baby Mama --There's No Doubt, Kate Will Be a SINGLE Mother

Once a deadbeat dad, ALWAYS a deadbeat dad ... so says one of Michael Lohan's baby mamas ... who says it's a lesson Kate Major is about to learn the hard way.

TMZ spoke with Kristi Horn - who has a 17-year-old daughter named Ashley with Lohan. Horn tells us Michael isn't just an absentee father ... he also owes her more than $300k in back child support.

And Horn says there's a ZERO PERCENT chance MiLo changes his ways with the new kid on the way.

"This is a freaking train wreck ... here we go again," Horn says.

"I just feel so bad for Kate. She’s going to be a single mother eventually ... he’s going to leave her to raise their child alone like he did to Dina and I."