You have not specified what kind of Guru was KAnchipurna of RAmanuja. Whether just a ShikshA Guru or more than that. That is, Mantra Guru or DikshA Guru.

Anyways, since a DikshA Guru is usually called a Guru, i am assuming he gave Mantra to RAmanuja.

Then this is really something that is unheard of.

First, of all, to be a Guru, one has to obtain right (AdhikAra) of all Mantras. But, a Sudra can not do so. Because there are restrictions imposed upon the kind of Mantras he can be initiated into. So, a Sudra can not be a Guru.

For example, a Sudra can not be initiated into Mantras that have Pranava, SwAhA in them. Plus there are other restrictions.<br>

Even, the case of a Sudra initiating another Sudra is proscribed for that reason:<br>

> Swadharma nirato bhutva srutvA dvija gurormukhAt |<br> Sarva
> siddhimavApnoti shighram devatva mApnuyAt ||<br> **Sudrah
> sudramukhAcchrutvA vidyAm va mantrameva vA |<br> GrihitvA narakam yAti
> dukkham prApnoti nishchitam ||** 
> .......
> One, following his own rules for righteous conducts (swadharma
> nirato), must get mantra from a Dvija Guru's mouth and thereby he attains
> all special powers and accomplishments and soon attains divinity. **A
> Sudra upon receiving mantra from a Sudra Guru without doubt suffers
> and visits hell**.<br> 
> **Vishnu YAmala Tantram.**

Although here the word Dvija (which may refer even to a Vaishya) is mentioned the commentator explains and proves that it refers only to Brahmins in this case. 

He quotes a NArada PancharAtra verse (viz: VidyamAne tu yah kuryAt tatra viparyama ... ) to show that even Kshatriyas and Vaishyas are not qualified to initiate a Brahmin. A Brahmin has to be usually a Guru for a Brahmin.

In HariBhakti VilAsa and Rudra YAmala it is stated the same viz: 

> Pratilomyam na dikshAyet

Or in other words, Sudra is not qualified to initiate others.

However, there is an exceptional case, when the ordinary rules fail to apply.

 Lord Shiva says to Mother Goddess, in the next given verse, that even a person who fails to meet all the normal standards of a Guru (that is one who is Guru-Lakshana-Hina) can still be one if he is the knower of  the Self or that of Brahman. <br>

> **Sarva lakshana hinohapi tattva gyAni guru smritah** |<br> TasmAt tattva
> videveha mukto mochak eva cha ||
> ......
> **One is still called a Guru if he is tattva-gyAni (self-realised), even
> though he might not have any other qualifications [that are otherwise
> required].** Only one who is a tattva-gyAni is liberated and only one
> who himself is liberated can liberate others.<br>
> **KulArnava Tantram 13.121.**

So, everything now depends on whether KAnchipurna was self-realised or not.