In reply to gethin_allen:
Dale Sailing (www.pembrokeshire-islands.co.uk) are pretty good. All their trips leave from Martin's Haven; you've got a reasonable chance of seeing them as you pass across Jack Sound, but the best option would be to go on a Grassholm trip. I'd go on the Dale Princess - it's slower, but you get more of a chance to see the wildlife, and are higher up than a rib, so get a better view. The boatmen on the Princess all know the area and it's wildlife pretty well.
It's difficult to predict dolphin movements; there might not be any sightings for a few weeks, and then a pod of over 100 will be spotted. In my experience there's a slight correlation between sightings and relatively warmer water - but look luck trying to identify in advance when that's going to happen. What you will see around St Bride's Bay (especially to the south) on the 20th April are plenty of Puffins, Gannets, Kittiwakes, and Razorbills, and (weather depending) Manx Shearwaters. When you go, look out for Gannets - if a group are circling round a particular spot and diving, it often indicates that there are dolphins or porpoise in that area - they stir up the fish, and the Gannets use the opportunity to get some easy food.
If you decided that you wanted a really good tour with seabirds instead, then one of their evening trips with an expert (quite often Anna or Steve Sutcliffe, who lived on Skomer for a number of years and both really know their stuff) would do the trick.
Is your friend prone to seasickness? Worth thinking about - although they don't run trips in the worst weather, the sea is rarely like a millpond round there (especially in April).
I don't know the companies based around Ramsey; I do know that there are reasonably regular sightings of dolphins around the island, but pods move around St Bride's Bay, and don't stick to one area.