Vince Neil Wasn't on Crashed Jet Because He Extended Birthday Bash in Florida

Vince Neil's birthday party in Florida may have been a lifesaver ... because it's the reason the Mötley Crüe frontman wasn't onboard a doomed private jet flight that killed his longtime pilot.

Vince's close friend, Lance Graves, tells TMZ … before Vince's private jet was involved in a fatal crash Monday in Arizona, Neil and his friends were in Florida celebrating his 64th birthday.

The rocker's girlfriend, Rain, was on the flight with her friend when it landed at Scottsdale Airport, lost control and crashed into a parked jet -- killing Vince's pilot -- but Vince was not.

Vince's friend says Rain had to be in Arizona to check in for a gig at the Royal Crown barrel racing competition, a horse event -- but Vince didn't need to be at the competition until Thursday, so Vince stayed behind in Florida to continue enjoying the beach.

As we first reported ... Vince's girlfriend suffered 5 broken ribs in the crash. She was traveling with a friend who was also injured, and the plane's co-pilot suffered injuries as well.

Vince's friend says Rain will no longer be competing in the barrel racing competition as she'll need time to recover from her injuries, both physically and emotionally. He says Vince's friends will give her the "time and grace" she needs to heal.

Mötley Crüe's manager previously told us Vince's heart goes out to the pilot's family and the band is brainstorming how they can help.

Vince's friend says the pilot, a 78-year-old Marine veteran, flew with Vince for many years and Vince felt a loyalty to hire him after he retired from commercial airlines and wanted to support him.