No start help

Hoping someone out there can help. Here's what happened: My bf & I were driving his 2001 BMW 325i on the hwy & all of a sudden, it decelerated & it acted like it was going to die, but quickly resumed speed & seemed ok. The day before the check engine light was on, but not when this incident happened. It was low on fuel, so 5 mins later, we pulled up to a gas pump, & the car died. He's driven the car with less fuel than what it had, so we don't think it just ran out of gas. He tried to restart the car, it started but immediately died. Tried to do this a couple times. We pushed it into a parking stall, added some fuel injection treatment added some gas, & ended up having to tow the car. The car sat for maybe 2 weeks. He tinkered with some stuff under the hood, & the car started up. Made 3 25 mile trips, then when he went to starting, it wouldn't start. Added some more fuel injection treatment & gas. Wouldn't start. Messed with some sensor under the hood. It started up. Drove it a short distance, less than 5 miles, probably twice, & it died. Now it'll try to start, but won't. He's tinkered with the same stuff under the hood, has fuel, checked fuses, took battery out, fully charged it, & still nothing. Can anybody please narrow it down, or offer some help. It would be greatly appreciated. Thanks in advance.