Yes, if they entered legally (even if they overstayed), marriage to a U.S. citizen allows them to adjust status from within the U.S.

Overstay is forgiven in immediate relative cases (spouses, parents, minor children of U.S. citizens).

However, misrepresentation at entry (e.g., saying they came as a tourist when intending to marry) could cause complications.

If married to a green card holder instead of a citizen, overstay becomes a bigger issue and may require consular processing with waivers.