Heidi Montag and Spencer Pratt's relationship has gone from heaven to hell-on-earth.
They are now each accusing the other of having affairs, RadarOnline.com has learned exclusively, with Heidi going so far as to hire a private investigator to spy on Spencer.
As RadarOnline.com exclusively revealed, Heidi is staying at the Malibu home of Cougar Zank, a Marine turned bodyguard and bit actor who had become close friends with Spencer.
Earlier this year, Pratt moved Zank into the Pacific Palisades home he and Heidi were renting, with Spencer claiming the two men were working on a new script together.
But when Heidi and Spencer split a month ago, Zank invited Heidi to stay with him at a home he rents in Malibu.
That made Spencer's hair stand on end.
He considered Cougar HIS friend, and he's convinced himself the only reason Cougar would have sided with Heidi in the split is that "he wanted her for himself," a well-placed source tells RadarOnline.com.
The source revealed that Heidi is just back from New York City and that Cougar accompanied her on the trip.
"They are definitely friends, and I don't know if there is anything more to it than that, but Spencer believes there is. He's certain there is."
The trip to Manhattan wasn't a social jaunt.
Heidi is trying to find a production company interested in backing her and former Hills co-star Jen Bunney in a new reality series about two single girls traveling the world.
She met with executives at ITV, the source says.
ITV is the company behind I'm A Celebrity - Get Me Out Of Here, which featured Heidi and Spencer as contestants exactly a year ago.
Heidi is serious about going it alone, the source says.
She is definitely following up on her petition for legal separation with a divorce petition.
"She's just getting organized on a few things, and then it will be filed," the source reveals. "She felt she had to get the separation petition done in a rush last week to stop the money flow."
Under California law, community property ends on the date of separation.
"Heidi is very concerned about the finances," the source says.
"Everybody knows she's made more money than Spencer. She needs to protect it."
One of Heidi's major concerns, the source reveals, is that Spencer has been seeing another woman since their split in May.
"She is very anxious to find out if her suspicions and the rumors are true, especially if he's spent any money on another woman.
"Heidi has hired a former member of the sheriff department, who now works as a private investigator, to look into it," the source reveals.
"It is really important to Heidi to find out if Spencer has been cheating on her."