Is a loan made to obtain a HELOC on a consumer's primary residence that is owned by a company in which the consumer is the principal owner a rescindable transaction?

No. The individual has no ownership interest in the property.

Ok - does it matter if the loan is for a consumer purpose?

No. Who is the borrower and what is the purpose?

The borrower is an individual and purpose is a HELOC for personal expenses. The home is owned by a company where she is the majority owner.

Then it is just a line of credit secured by real property and subject to 1026.6(b).

But does the individual have a right to rescind? I know the individual is not an owner, but in an indirect way, they are.

No they are not. The entity owns the property. Who owns the entity is irrelevant.

Part by part
1026.15 (a) Consumer's right to rescind.
(1)(i) Except as provided in paragraph (a)(1)(ii) of this section, in a credit plan in which a security interest is or will be retained or acquired in a consumer's principal dwelling, each consumer whose ownership interest is or will be subject to the security interest shall have the right to rescind: each credit extension made under the plan; the plan when the plan is opened; a security interest when added or increased to secure an existing plan; and the increase when a credit limit on the plan is increased.

security interest is or will be retained or acquired in a consumer's principal dwelling - dwelling is owned by company - NO
each consumer whose ownership interest is or will be subject to the security interest - business has security interest - NO