If your colleague lacks experience writing tests maybe he or she is having difficulty testing beyond simple situations, and that is manifesting itself as inadequate testing. Here's what I would try:
- Spend some time and share testing techniques, like dependency injection, looking for edge cases, and so on with your colleague.
- Offer to answer questions about testing.
- Do code reviews of tests for a while. Ask your colleague to review changes of yours that are exemplary of good testing. Look at their comments to see if they're really reading and understanding your test code.
- If there are books that fit particularly well with your team's testing philosophy give him or her a copy. It might help if your code review feedback or discussions reference the book so he or she has a thread to pick up and follow.
I wouldn't especially emphasize the shame/guilt factor. It is worth pointing out that testing is a widely adopted, good practice and that writing and maintaining good tests is a professional courtesy so your team mates don't need to spend their weekends at work, but I wouldn't belabor those points.
If you really need to "get tough" then institute an impartial system; nobody likes to feel like they're being singled out. For example your team might require code to maintain a certain level of test coverage (able to be gamed, but at least able to be automated); require new code to have tests; require reviewers to consider the quality of tests when doing code reviews; and so on. Instituting that system should come from team consensus. If you moderate the discussion carefully you might uncover other underlying reasons your colleague's testing isn't what you expect.
