The Skydivers

Harry (Anthony Cardoza) and Beth (Kevin Casey) Rowe run a small skydiving facility in an unnamed desert town. One day, a woman named Suzy Belmont (Marcia Knight) and her brother Phil (Phil Triola) comes around, claiming to be looking for the Rowe's plane mechanic Frankie Bonner (Titus Moede). Beth claims that Frankie was fired for being drunk on the job, but feels that Suzy didn't come down just to see Frankie. As she walks away, Beth can't help but feel that her husband is having an affair with this floozy. It turns out that Harry is, but is still keeping it a secret from his wife. One evening, they receive a letter from Harry's friend, Joe Moss (Eric Tomlin). Joe wants to visit and is looking for a job. Beth recommends to Harry that Joe could easily fill Frankie's position, to which Harry gives his consent that it's okay with him.
Soon after, Frankie returns to the skydiving school, but is caught by Harry attempting to sabotage a plane. After being accosted, Frankie demands Harry stay away from Suzy. Harry agrees, but warns Frankie that he'll break both his legs if he comes back to the facility.
Eventually, Joe Moss arrives, and is greeted by Harry and Beth. The little reunion is soon short-lived when a man named Pete (Paul Francis) arrives, and wants to do a dangerous skydiving stunt. Harry warns Pete that this could get the FAA involved with them, but Pete claims he can do it. However, before he can pull his chute, Pete panics and plummets to the ground. This incident causes the FAA to shut down the Rowe's facilities. Harry drives into town to a local bar, where after drinking a beer, he encounters Suzy outside. She attempts to woo Harry, who instead calls her a 'broad.' A fight breaks out between the two of them, in which Harry eventually walks away.
Incensed at what Harry said, Suzy then plots revenge by convincing love-struck Frankie to help her put acid in Harry's parachute when he does a skydive jump once the skydiving facility reopens. Frankie is unsure, but still goes along with it.
Eventually, the facility reopens, and numerous people come out to see the skydivers. However, trouble is brewing inside the Rowe's home when Harry suspects that Joe may be having an affair with Beth. Harry soon makes up with his wife, and Joe stops attempting to get acquainted with Beth. However, on the evening of a particular night jump, Suzy and Frankie sneak into the hangar and pour acid on Harry's parachute. When the jump is made, Harry's chute rips and he plummets to the ground.
Some witness report seeing Suzy and Frankie running away from the preparations room. Joe gets in his car and soon after catches up to them. However, some men from the FAA give chase in a plane and in a car. In spite of the lack of any direct evidence that they were responsible and no legal proceedings, the two are immediately gunned down without warning by the authorities as they drive away from the facility.
In the aftermath, Joe takes his leave of Beth, who is giving up running the skydiving facility. As Joe drives on his way, Beth takes her own leave of the facility. The curiously grim end credits feature names and head shots of the actors set to total silence.

Question: Who suspects Joe is having an affair with Beth?
Answer: Harry