Giving a professional reference to anyone whom you worked with should be identical to giving an eulogy for someone dead – it must be heaped with praises and specific events to highlight the best thing the person has done. Same rule applies for the retirement speech for anyone as well. Never give a reference to anyone whom you have not worked with long enough to highlight their strength and achievements and be very specific about what you say. Keep the language simple (not verbose) and talk about impact. Be good on what you write and keep it crisp and short.