According to the information currently entered, they can't be compared at this time, because one has an autosomal DNA test and the other has a yDNA test. There are 3 main types of DNA tests, autosomal DNA test, mitochondrial DNA test, and Y-chromosome DNA test, and you can only compare tests that are the same type.
Your options are to either have Joe Janke take a similar yDNA test (which may not be conclusive), or have Peter take an autosomal test (I'm guessing you have more influence over Peter than Joe!). If the relation is distant, then it may still be hard to conclude what the relation is. If Joe takes a yDNA test, then a negative match would be a conclusive non-match. A positive match (same haplogroup and exact match on STR's) would be strongly suggestive, but not conclusive.
To compare autosomal results, Joe would need to upload his raw DNA data to GEDmatch, and so would Peter. Then they could be compared. Failing that, Peter would have to take the Ancestry DNA test only, and look for Joe in the results on Ancestry.com.
Correction! Joe's father is unknown, and he apparently took his last name from his mother, Donna Janke, so yDNA won't help at all. And that means you are looking for a connection to Donna, not Joe.
