Jessica Surprised by Pete Wentz

(James Devaney/WireImage)

While her future brother-in-law Pete Wentz was celebrating his bachelor party, Jessica Simpson had a Thursday night out with three girlfriends at Hennessy’s Jazz Night at Teddy’s in the Hollywood Roosevelt Hotel.

Little did the newly single bombshell know that Pete, who had been dining with family and friends at Mr. Chow, would choose the same hotel for an after-hours party with 20 of his pals.

But before Pete, dressed super-casual in black jeans, gray tee and a cutoff black hoodie, headed upstairs to the Marilyn Monroe suite, he stopped in to say hello to Ashlee's big sister.

“When Jessica saw him, she was completely surprised,” a fellow partygoer tells OK!. “All the girls yelled ‘Pete,’ and Jess jumped up and ran over to him and gave him a big hug.”

Though he did the gentlemanly thing by popping by to see Jess, the onlooker adds, “He looked like he was in a rush.”

After he left and went upstairs where the Hennessy was flowing, Jessica let him do his thing and continued partying and dancing all night.