The target dies painfully, unless it's an incredibly obese person.
If it's an obese person, they're still in an incredible amount of pain, but might not die.
Things expanding within a person's stomach falls under the category of 'things you never want happening to you'. I'm also assuming this is a 'matter of seconds' thing and not 'matter of hours' because magic, and its more destructive that way. Let's figure out what happens. First, the stomach bursts open. Literally bursts open. A human stomach is the size of a the person's fist, and is actually pretty good at expanding / condensing food, usually. The problem here is that if the watermelon isn't chewed, the fact that the inside of the watermelon is basically water does not help here, as all that easily-disposed-of water is inside the fairly tough rind. So the first thing to go is the stomach. Then the nearby organs.
The nearby organs here are also important ones - liver and kidneys. (Also some more significant parts of the digestive system, but you can technically live without a pancreas.) Human organs aren't meant to take sudden impacts - that's what the skin and muscles are for, to stop that from happening. So the internal organs take shock damage, and also if this is a particularly large watermelon (and a small human), it might also be big enough to burst through the skin and chuck these very important organs out. Even if this doesn't happen, these organs are still pretty much mush. Oh, and now that the stomach is split open, you have stomach acid (gastro acid - pH of ~2.5) leaking all over the interior of his stomach.
The heart is relatively safe in this scenario, relatively safe here meaning that it probably doesn't get squished like a grape. If the person's lungs are full, they can help take some of the impact, the heart is pretty tough, and the stomach isn't that close. It'll still feel like a jackhammer hit it, but it'll keep pumping, so the target won't die from that, though it might be preferable to what comes next.
There will also be massive internal bleeding, because the abdomen now has a watermelon in it, destroying everything, so there's that. (The stomach acid doesn't help either.) I'd guess eventual death of blood loss as the target bleeds out from the abdomen and / or suffering from shock. In the event that the subject survives the initial injury, the loss of kidneys and liver will mean the subject dies from eventual blood poisoning because now his liver is missing. So, ah, not good.
Theoretically, if the target was obese enough, the fat may be able to act as a buffer, so when the stomach bursts (yes, it will still do that if it happens spontaneously), the damage might be mitigated, and emergency surgery might be able to keep the target alive. Though the level of obesity I have in mind would probably kill the target on its own from multiple heart attacks.