The person who told you that the Spanish here is grammatically incorrect was wrong.

This is a verse from the Bible, Ecclesiastes 11:9. The quotation you have is from the Spanish version of the Jehovah's Witnesses' bible. Here it is on their website:

http://wol.jw.org/en/wol/b/r4/lp-s/21/11

It is listed on the site as:
"Traducción del Nuevo Mundo de las Santas Escrituras (Con Referencias)
Una traducción revisada basada en la versión de 1984 en inglés, pero consultando fielmente los antiguos textos hebreo y griego —1987—"
http://wol.jw.org/en/wol/d/r4/lp-s/1001060118

In this verse it seems to be a very accurate version of the original Hebrew text, which can be seen here with an English translation:

http://www.mechon-mamre.org/p/pt/pt3111.htm

So very literally, the Spanish means "Let your heart do (haga tu corazón) good (bien) to you (te)". The "te" has to be appended to the end of the subjunctive verb, "haga". It is not a reflexive pronoun, but simply an indirect object pronoun ("to you").

The English version I have posted is from the New Revised Standard Version of the Bible:

http://www.biblegateway.com/passage/?search=Ecclesiastes 11&...

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Note added at 27 mins (2013-07-17 21:43:46 GMT)
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Just to develop the grammatical explanation a little further, "tu corazón" is the subject of "haga", which is subjunctive and means "let it do" or "may it do". "Bien" is the direct object of "haga", and can be translated as "good" (noun) or "goodness" or "blessing".

"Te", as I have said, is the indirect object and expresses the person to whom good (or whatever) is done. Any appropriate pronoun could have been used, depending on the meaning: "Hágame bien", let it do good to me, "Hágale bien", let it do good to him/her/it, "háganos bien", let it do good to us, and so on.

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Note added at 55 mins (2013-07-17 22:11:34 GMT)
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On "hágate", see the following explanation from the Real Academia Española. This verb, "haga", is what they call a "subjuntivo exhortativo". "Clíticos" means unstressed pronouns, like "te":

"c) Los clíticos se posponen a las formas de imperativo y a las del subjuntivo exhortativo afirmativo: Hazlo; Ponételo; Dígannoslo; Hágase la luz. Es vulgar anteponer los clíticos al subjuntivo exhortativo cuando este no depende de otro verbo: X «¡Se callen, carajo, no es hora de conversa!» (FnGómez Viaje [Esp. 1985]); debe decirse cállense. Sin embargo, la anteposición es obligada cuando el subjuntivo va en forma negativa o depende de otro verbo (explícito o implícito): No lo hagan; Les ordeno que se callen; Que se vayan ahora mismo."
http://lema.rae.es/dpd/?key=pronombre
(Diccionario panhispánico de dudas, Pronombres personales átonos, 3)

So if it had been "Que haga", which would be more normal, or at least less formal, in modern Spanish, "te" would have preceded "haga": "Que te haga".