How fast is it safe to lose weight?

I recently joined, and introduced my boyfriend to the site too. We're both within the healthy BMI range but would like to get a bit lower in the range, but he's especially keen to lose weight FAST, while I'm not sure that's wise (I mean, I would love to get 10lbs off in a week even if it meant suffering, but from what I read that would mean losing muscle mass, which would defeat the point).

We both exercise a reasonable amount. I go to the gym for 30-60 mins a day and aim to net around 1250 calories a day and am more or less on target most days. He cycles to work and back (1 hour total), and often goes to the gym too, so can sometimes burn upwards of 700 extra calories a day, which sometimes leaves him way under his goal of net 1500 calories (which I know is already low for a guy). He gets all excited when the myfitnesspal weight estimator tells him "if every day was like today" he'd lose like 5kg (11lb) in 5 weeks. But is it good to lose so fast when you're already within a healthy BMI? He doesn't seem to 'get' that he might end up losing lean muscle mass (he's still in the mindset that weight lost is weight lost).

He always EATS at least the 1500 calories, it's just that he exercises so much that his net calories can sometimes be low - does this mean he can safely lose the weight quite quickly if he's doing it by burning rather than by going on a starvation diet? If it's not healthy it would be great if you could give me some medical facts to back up my argument, because at the moment I think he thinks I'm just trying to sabotage his weight loss.