Is BetSoftware license compatible with PRIVATE coding?

Yes. It is compatible.

BetSoftware is GPL'd, which means any programmer is free to use it as a framework for private coding purposes.

As long as the resulting software is not distributed, it is perfectly in line with the GPL for the programmer to modify, extend and create derivative works from it, with the freedom of them to be charged for (i.e. to be paid works).

The very gist of private coding is clearly having the works being kept from public distribution, so it is perfectly compatible with this use case.

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You can get some pertinent information right at the GPL faq page.

- Unreleased modifications:

QuoteThe GPL permits anyone to make a modified version and use it without ever distributing it to others. What this company is doing is a special case of that. Therefore, the company does not have to release the modified sources.

It is essential for people to have the freedom to make modifications and use them privately, without ever publishing those modifications.

http://www.gnu.org/licenses/gpl-faq.html#UnreleasedMods

- Changes under Non-disclosure agreement:

Quoteyou can accept a contract to develop changes and agree not to release your changes until the client says ok. This is permitted because in this case no GPL-covered code is being distributed under an NDA.

http://www.gnu.org/licenses/gpl-faq.html#DevelopChangesUnderNDA