Yeah, the idea of Freddie being ill gave the Live Aid performance a lot of dramatic power as well as musical, so I can see why they used it in the film even though it wasn't strictly accurate.
Now Mercury may not have been officially diagnosed, and wasn't actively dying, but he had to have been living in fear of an early death at that point. He was a sexually active gay man living during The Plague years, watching his friends and peers sicken and die one by one, and wondering if every cough or fever meant the beginning of the end for him. If he went out on that stage and sang like it was the last time, it could have been because he knew that was a strong possibility.
The film is a biopic. It should not take so many liberties with the facts and dates.
If Freddie was worried about AIDS, they should have presented that factually or nearer to the truth. If they had presented his fear of aids and feeling weak before Wembley, it would have made much more sense. As it was presented, it is a slap in the face to Freddie Mercury, and it tarnishes Queen as a band.