For a friend with a good sense of humour, a roast is one way to honour him for a special occasion. A roast involves friends getting together to perform jokes at the guest of honor's expense. The key to a successful roast is striking a balance between comedy and criticism; too much venom and the roast can feel mean-spirited, not enough and the jokes won't land. The nature of the jokes and the subject matter should be catered to the audience.
Roasting someone's work ethic is generally tame and therefore good subject matter for a kind-natured roast. This topic is especially fitting to roasting someone who is a co-worker as well as a friend. Whether the person is a very successful business worker or currently unemployed, this topic can be approached to match your friend's work situation. Remember to finish the roast with a few strong compliments to balance out the seemingly negative jokes, or allow the guest of honour to make a few jokes at the expense of the roasters.
Romantic or platonic, the relationships the person has can be fodder for many jokes. However, there are some topics that might be inappropriate for the roast. For example, if a relationship ended poorly and the wound from this loss is not fully healed, do not attempt making a joke on this topic unless you are certain the friend has an extraordinarily strong sense of humour. Another consideration is who will be in the audience. You shouldn't insult an innocent member of the audience who might not be involved with the evening's roast.
Comparisons are easy ways to come up with jokes for the roast. To start, take a personality trait or hobby that the guest of honour is prone to. Then think of something else that might do this particular act as well. For example, if the guest of honour eats junk food, you may say they "hold more trash than a compactor." If the person is very fair-complected, you may say that if he "got any paler he could light up the night sky." Of course, your comparisons can be more adult-themed depending on the audience. It might be a smart idea to test out your material in advance on a few friends to gauge their reaction.
Be they physical or behavioural, weaknesses can be material for your roast. These weaknesses could be everything from weak academic history to a bad habit like smoking. This works best when roasting someone you are very close with. Otherwise you will need to do some research to make the jokes as relevant as possible.