There are two questions here. The first is easily answered: Is “a Euler graph” correct? No. It should be “an Euler graph”.
The other question is more tricky: If the source of a cited piece of text contains an error in the grammar or spelling, or some other obvious error, should I correct it? First, you should try to reference the original source; it may be that the error was introduced in the secondary source that you first used. If so, quote the original source. If the original source has the error, then it is arguably appropriate to quote it verbatim but with “[sic]” appended. That is what I would do. However, there are different ideas about this. One is that peppering quoted text with “[sic]” is a slippery slope; just copy it as it stands. The other is simply to correct obvious mistakes without comment. That makes reading easier but offends the principle of historical precision. In the end, the decision is yours or the editor's, depending on who holds sway here.
(Added in edit) In this case, the original source (thanks to Gonçalo) is a reputable edited journal and bears the error. So some explanation is called for. My guess is that the author and/or editor thought that Euler was pronounced “yooler”, in analogy to eulogy, for which of course the form a eulogy is correct.