Name the fallacy

Again we have an unsupported claim or opinion. This gets into the debate as to whether offense is given or taken. For example, does one's intent matter? I can't imagine that someone using the adjective "cancerous" would have any mocking intentions, as even people with cancer would agree it is a horrible disease that destroys. But "cancerous" is not the same as terms such as "retarded," or "mental" (not that you claimed it was, just covering this base). Other terms that are not as clearly malicious as "cancer" that are used to associate with something bad associate more negativity to the term. Logically, they can be considered a Weak Analogy .

Using the definition of a term in more than one way is an Equivocation Fallacy. But, there is not enough information given here to suggest they used the term in more than one way. 

However, your assumption that they were doing it to mock those with cancer might be a Strawman Fallacy as it could be a misrepresentation their intentions. Or it could be a Non-Sequitur Fallacy as your conclusion that they used the word "cancer" to mock cancer patients doesn't necessarily follow the premise that it was a poor presentation. 

"The presentation given tonight was cancerous" equals mocking those with the condition."

So the argument form goes something like:

"Comparing anything to cancer mocks those with the condition.

You compared X to cancer.

Therefore, you are mocking those with the condition."

It looks like it makes sense internally, but it does require an assumption of bad faith. People often do this by failing to realise that there are multiple interpretations of certain words (e.g. " depressed ", " pissed ") and assume the interpretation they are most familiar with/that is most convenient to their argument.

" Cancer " also refers to something that spreads rather alarmingly, with severely negative effects. (e.g. "the alt-right's influence is growing like a cancer in the darkest corners of our internet."

As Jason Mathias mentioned already, I wager it could either be a strawman (though it doesn't actually misrepresent the statement, it simply assigns malicious intent to it) or a non-sequitur (just because you used a word to describe something doesn't mean automatically you're insulting those who may suffer from it, if it describes a medical condition).

It's important to note this could also be a matter of opinion. While you can't assert - fairly - without evidence that the person is "mocking" others, it could still be seen as offensive.