Is your friend a certified mechanic?? A cell in a battery going bad will give the symptoms you describe. I just replaced the battery in my 04 for this exact reason. It would be fine for a whole day, let it sit overnight and it had to be jumped. Once jumped I could drive wherever I wanted for a day without worrying whether it would start or not. Then without fail I had to jump it again the next morning everything. The dead cell will take the charge for a while but it will drain faster from things such as the 12 volt constant to the stereo and power to the security system. It's still taking a charge it's just taking a lot less to use it up since the cell is dying. Would take you ten minutes to drive to a parts store and have them test it. What's it gonna hurt?
04 3.9l Convertible- Daily driver
94 5.0l Convertible- Work in progress
92 5.0l Hatchback- Another work in progress