As Mike Scott notes, we don't know yet. Our current exoplanet survey techniques are so strongly biased in favor of planets that are close to their sun (and therefore likely to be tidally locked) that they don't yet allow reliably estimating the fraction of all planets that orbit further out (and therefore probably aren't locked).
That said, you hypothetical aliens probably don't care about the total ratio of tidally locked to non-locked planets, anyway. What they care about is the fraction of life-bearing planets that are tidally locked to their star. That's, of course, even harder to estimate using current data (since we haven't even detected any life-bearing planets other than Earth yet), but we do have the following suggestive observations:
By far the majority of all stars are small red dwarfs, much smaller than our Sun.
Our current exoplanet surveys do seem to indicate that such small stars are capable of having Earth-like planets within their habitable zone.
The smaller a star is, the closer to it its habitable zone lies, and thus the more likely a planet orbiting in that zone is to be tide-locked.
Thus, given how common red dwarfs are, there's statistically likely to be a rather large population of planets around them that would be both tidally locked and, by their size and orbital radius, capable of supporting liquid water on their surface. If we assume that those planets aren't significantly less likely to bear life than otherwise similar non-locked planets around larger stars, then there's a good chance that the majority of all planets bearing water-based life are indeed tidally locked.
Of course, that's a big assumption, but it's one we can't currently either prove or disprove. Furthermore, your hypothetical scenario implicitly provides are least one extra (hypothetical) data point in its favor: the existence of your alien civilization from a tidally locked planet.
All that said, there's a couple of things that may reduce the element of surprise for your alien survey team spotting the Earth:
First, if they're surveying for habitable planets around Sun-like stars in the first place, they're presumably smart enough to do the math and figure out that such planets are not likely to be tide-locked to their star. If they're so convinced that this makes life impossible, why bother even surveying such stars?
Second, given the observed existence of "hot Jupiters" (and their apparent prevalence, which of course may also be partly due to survey bias, since they're the easiest kind of planets to detect), it's reasonably likely that a significant fraction of all theoretically habitable bodies around small stars may actually be moons of large gas planets.
While such moons will almost certainly be tide-locked, they'll be locked to their parent planet, not to the star, and will therefore have a more or less Earth-like day-night cycle. If life on planets around small stars is possible at all, it's likely that such moons can also support it equally well (or better), and thus your alien survey team ought to be aware of the possibility of life on such worlds. From there, it's not a huge leap to suppose that freely rotating planets around large stars could also be viable places for life.
Ps. See also: Habitability of red dwarf systems and Habitability of natural satellites on Wikipedia