Article 6(1)(f) gives you a lawful basis for processing where:

“processing is necessary for the purposes of the legitimate interests pursued by the controller or by a third party except where such interests are overridden by the interests or fundamental rights and freedoms of the data subject which require protection of personal data, in particular where the data subject is a child.”

1. Purpose test: are you pursuing a legitimate interest?

Yes-Debt collection, plus warning others about bad debtors

2. Necessity test: is the processing necessary for that purpose?Yes

3. Balancing test: do the individual’s interests override the legitimate interest?No