Fretless bass is great, but it's not something you dabble with or do for fun. Tell him to get a lined bass, that way he can have a visual reference to match to the tones. Noodling around on a fretless bass is easy and fun, but getting good intonation takes practice. Most non-musicians will not notice unless it's really bad, but musicians will pick up on it straight away. I have only ever dabbled on unlined fretless basses that my brother has owned and my intonation is terrible. My brother practised long enough to have excellent intonation, as I only ever play bass for 5 minutes here and there for a laugh, it's not something I aspire to.
Your friend should not be afraid of fretless bass, but I think it will be easier to learn on a lined fretted. If he cannot get the kind of bass he wants with a lined fretboard, then he will just have to learn without lines, which is not impossible. If he is hoping to do it as a quick and easy side-piece to his regular bass playing, he should not expect to be much good at it. It takes time and practice, but like most other things in music, the journy of learning it is as much fun as the destination of being good at it.
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